Plumo Ultralight Sync

Introduction to Plumo Ultralight Sync, it's core concepts, architecture, process, and implementation.

---

What is Plumo Ultralight Sync?

Plumo is a zk-SNARK based system that allows mobile and resource constrained nodes on the Celo network to sync to the Celo blockchain faster and with less data. It accomplishes this by using zero-knowledge proofs, which allow the quick verification of the chain syncing computation without having to run it locally.

Background

Celo is a mobile-first blockchain platform with a mission to enable the conditions for prosperity across the world. This means that may of Celo and the Valora wallet's first users will be in emerging markets. In these countries, while mobile phone penetration has significantly increased, data plans and device hardware still lags. This means that for average users, data is a scarce commodity and devices are underpowered in compute relative to the latest and greatest iPhone or Android devices.

If these users are to trustlessly sync and interact with a blockchain network, they'll need to run a light client on devices. However, traditional light clients in proof of work (PoW) blockchains need to download every block header, which grows linearly with new blocks. Over time, this becomes untenable, and even light clients take minutes to sync.

Proof of Stake Light Clients

Celo uses a proof of stake (PoS) consensus sytem, enabling it to offer a better light client sync experience, right off the bat:

1. First, Celo light clients only need to download epoch block headers, which contain information about the current validator set. The validator set in Celo changes once every epoch, roughly about once per day. This means instead of downloading a block header for every block on the Celo network, they can just download one a day.

2. Second, Celo uses BLS signatures to aggregate validator signatures. Similar to Tendermint consensus, Celo's IBFT consensus mechanism requires a quorum of 2/3 or more validators to sign each block to commit it to the blockchain. Instead of having each of these signatures be separately stored on-chain, Celo uses BLS signatures which can combine them into one signature per block.

These two innovations reduce the time and data required to sync significantly, but over time, sync time continues to increase. One option is to checkpoint the chain ever so frequently so that clients do not need to sync from the genesis block. This certainly alleviates the challenge, but checkpointing requires getting consensus and agreement from the community, which can take time. What if there were a way to "checkpoint" cryptographically?

Enter Plumo

This is where Plumo comes in. Plumo zk-SNARKs allow light clients to quickly verify transitions from epoch X to epoch X + Y, where Y is hundreds of epochs. This means that instead of verifying each epoch, light clients can "hop" from SNARK to SNARK until they reach the latest proof. From there, they can download the remaining epoch block headers to sync to the head of the chain.

The Plumo proofs are generated by a prover that can be run by multiple nodes across the Celo network. Proofs are then requested by light clients on the modified light client protocol. Light clients are able to verify proofs using the verification key that is included with the binary.

Using Plumo, light clients can drastically reduce the amount of data and time required to trustlessly sync with the Celo blockchain. But one more step is required for Plumo to be live on the Celo network -- the Plumo MPC setup.

Plumo MPC Setup

In order to generate the prover and verifier keys for the Plumo SNARK circuit, it is necessary to run a multi-party computation (MPC).

An MPC is a cryptographic mechanism for different parties to jointly perform a computation. SNARK circuits require a “trusted setup” where a shared secret is used to generate public parameters that can be used to prove and verify SNARKs. If one person ran this setup, then they could potentially prove incorrect things by exploiting a backdoor in the circuit. However, with an MPC, this setup process is split amongst tens or hundreds of contributors, and if even one of the participants is honest (keeps their inputs private), then the system will be secure.

In the case of the Plumo Ceremony, this collective computation will be a series of joint actions done by a group of participants from within the Celo community and beyond.

How to Contribute

The Plumo MPC setup is broken up into two phases:

• Phase 1 - Powers of Tau
  • The output of this phase can be used for either Groth16 circuits up to the size of the phase, or universal proving systems such as PLONK and Marlin.
• Phase 2 - Plumo circuit
  • In this phase, the participants contribute to the Plumo circuit keys, which would be used by provers to create proofs and verifiers to verify them.

Phase 1 will take place starting early December 2020, and continue until January 2021. It will consist of multiple rounds of 6–10 contributors each running the Plumo setup for approximately 36 hours. While much of the activity is passive and involves simply running the computation continuously, contributors should not expect to use their machines for other intensive activies through the duration of the setup.

Phase 2 will commence roughly a month after Phase 1.

Contributors will also receieve a gift for the time and efforts, and be recognized for their contributions to the Celo network.

Pre-requisites

You can run the contributor software locally or on cloud VMs, but desktop machines are preferable. The minimum machine requirements are detailed below:

Machine requirements:

• CPU model newer than 2016
• Minimum processor requirements:
  • 2.6GHz, 6 cores, 12 threads, OR
  • 2.3GHz, 8 cores, 16 threads, OR
  • 3.6GHz, 4 cores, 4 threads
• Operating system: Linux, macOS, Windows
• Recommended internet connection speed: 10Mbit upload

With these kind of machines, participiation in the setup should take around 30 hours, potentially a bit more or much less, depending on your specific hardware and internet connection.

Running the Setup

Phase 1

The instructions below are for the first phase of the Plumo setup, Powers of Tau.

Generate your address

The first step to participating is generating your Celo address in a secure location.

• Obtain the generate binary:
  • Option one - compile the generate binary:
    • Install Rust 1.49 using rustup install 1.49.0. If you don't have rustup installed, follow the instructions in https://rustup.rs/. If you're using an Apple M1 machine, install the beta version of Rust using rustup install beta.
    • git clone https://github.com/celo-org/snark-setup-operator
    • cd snark-setup-operator
    • git checkout ac3d5603256fc250500e00bae21ba646dd316d6f
    • cargo build --release --bin generate
    • Now you can use ./target/release/generate
  • Option two - if you prefer using a precompiled binary, download the generate binary corresponding to your OS from here.
• Use b2sum to check the hash of the binary against the hash in the download page by running b2sum FILE, where FILE is the contribute binary name for your OS. If you don't have b2sum installed, you can download it from https://github.com/dchest/b2sum, for example.
• Run it in a command line - navigate in the command line to the relevant folder - and follow the instructions.
  • When asked to Enter some entropy for your Plumo seed:, you can use any source of entropy.
  • Make sure to save your passphrase - you will need it later.
  • If you are using a USB thumbdrive, you can use the flag --keys-file KEYS_FILE, passing in the respective file path to save the key on the drive itself.
• Send the address generated to plumo@celo.org.
• Keep the resulting plumo.keys file. Ideally, it will be stored in a location that's easy to destroy afterwards, such as a USB thumbdrive.

Run the Contributor software

Next you'll obtain the contribute binary and begin contributing to the Plumo setup.

cLabs is running the coordinator server, which has a list of approved participant addresses. Additionally, cLabs is running a few verifiers that verify contributions on-demand, allowing the setup to make progress.

• After receiving confirmation from cLabs, obtain the contribute binary:

  • Option one - compile the contribute binary:
    • Install Rust 1.49 using rustup install 1.49.0. If you don't have rustup installed, follow the instructions in https://rustup.rs/. If you're using an Apple M1 machine, install the beta version of Rust using rustup install beta.
    • git clone https://github.com/celo-org/snark-setup-operator
    • cd snark-setup-operator
    • git checkout ac3d5603256fc250500e00bae21ba646dd316d6f
    • cargo build --release --bin contribute
    • Now you can use ./target/release/contribute
  • Option two - if you prefer using a precompiled binary, download the contribute binary corresponding to your OS from here.

• Use b2sum to check the hash of the binary against the hash in the download page by running b2sum FILE, where FILE is the contribute binary name for your OS. If you don't have b2sum installed, you can download it from https://github.com/dchest/b2sum, for example.

• Run it as follows, assuming your keys reside in KEYS_FILE:

  • Windows users: ./contribute-windows.exe --keys-file KEYS_FILE
  • macOS users: ./contribute-macos --keys-file KEYS_FILE
  • Linux users: ./contribute-linux --keys-file KEYS_FILE

  For example, assuming you're using Linux and your keys reside in /mnt/plumo/plumo.keys, you'd run: ./contribute-linux --keys-file /mnt/plumo/plumo.keys.

• You will be asked for your passphrase - enter the same one from earlier.

  • Follow the same process from earlier when prompted for additional entropy.

• Wait until you see 0/256 on the progress bar. This means that your contribution has started, and you are succesfully running the contributor binary.

Once this is running, you can leave the machine running -- no direct action is needed. This will run for about ~36 hours, after which the software will terminate running and you will have finished contributing to the Plumo setup!

Phase 2

The instructions below are for the second phase of the Plumo setup, the Plumo circuit.

Generate your address

The first step to participating is generating your Celo address in a secure location.

• Obtain the generate binary:
  • Option one - compile the generate binary:
    • Install Rust 1.52 using rustup install 1.52.1. If you don't have rustup installed, follow the instructions in https://rustup.rs/. If you're using an Apple M1 machine, install the beta version of Rust using rustup install beta.
    • git clone https://github.com/celo-org/snark-setup-operator
    • cd snark-setup-operator
    • git checkout 33717c3b0869c605e6c3627446e916f268712e00
    • cargo build --release --bin generate
    • Now you can use ./target/release/generate
  • Option two - if you prefer using a precompiled binary, download the generate binary corresponding to your OS from here.
• Use b2sum to check the hash of the binary against the hash in the download page by running b2sum FILE, where FILE is the contribute binary name for your OS. If you don't have b2sum installed, you can download it from https://github.com/dchest/b2sum, for example.
• Run it in a command line - navigate in the command line to the relevant folder - and follow the instructions.
  • When asked to Enter some entropy for your Plumo seed:, you can use any source of entropy.
  • Make sure to save your passphrase - you will need it later.
  • If you are using a USB thumbdrive, you can use the flag --keys-file KEYS_FILE, passing in the respective file path to save the key on the drive itself.
• Send the address generated to plumo@celo.org.
• Keep the resulting plumo.keys file. Ideally, it will be stored in a location that's easy to destroy afterwards, such as a USB thumbdrive.

Run the Contributor software

Next you'll obtain the contribute binary and begin contributing to the Plumo setup.

cLabs is running the coordinator server, which has a list of approved participant addresses. Additionally, cLabs is running a few verifiers that verify contributions on-demand, allowing the setup to make progress.

• After receiving confirmation from cLabs, obtain the contribute binary:

  • Option one - compile the contribute binary:
    • Install Rust 1.52 using rustup install 1.52.1. If you don't have rustup installed, follow the instructions in https://rustup.rs/. If you're using an Apple M1 machine, install the beta version of Rust using rustup install beta.
    • git clone https://github.com/celo-org/snark-setup-operator
    • cd snark-setup-operator
    • git checkout 33717c3b0869c605e6c3627446e916f268712e00
    • cargo build --release --bin contribute
    • Now you can use ./target/release/contribute
  • Option two - if you prefer using a precompiled binary, download the contribute binary corresponding to your OS from here.

• Use b2sum to check the hash of the binary against the hash in the download page by running b2sum FILE, where FILE is the contribute binary name for your OS. If you don't have b2sum installed, you can download it from https://github.com/dchest/b2sum, for example.

• Run it as follows, assuming your keys reside in KEYS_FILE:

  • Windows users: ./contribute-windows.exe --keys-file KEYS_FILE
  • macOS users: ./contribute-macos --keys-file KEYS_FILE
  • Linux users: ./contribute-linux --keys-file KEYS_FILE

  For example, assuming you're using Linux and your keys reside in /mnt/plumo/plumo.keys, you'd run: ./contribute-linux --keys-file /mnt/plumo/plumo.keys.

• You will be asked for your passphrase - enter the same one from earlier.

  • Follow the same process from earlier when prompted for additional entropy.

• Wait until you see 0/256 on the progress bar. This means that your contribution has started, and you are succesfully running the contributor binary.

Once this is running, you can leave the machine running -- no direct action is needed. This will run for about ~36 hours, after which the software will terminate running and you will have finished contributing to the Plumo setup!

Publish your attestation

The contribute binary produces a plumo.attestation file that contains a signature with your address. After your participation finishes (and not before!), please post it (here)[https://github.com/celo-org/plumo-ceremony-attestations] by creating a new issue! If you use precompiled binaries, be sure to mention you've verified the binary hashes posted on the releases page match the downloaded files.

If you don't have access to the plumo.attestation anymore, that's still OK, you can fill in the other details in the issue.

Troubleshooting

This section contains some common issues contributors may run into while running the setup software. If you have any questions, please reach out on the #plumo channel in the Celo Discord server. It is recommended that you have another device to be able to reach out in case of persistent issues.

Permissions:

• You may need to change the permissions on both the generate and contribute files to be able to run them. For example, on macOS, you can run chmod u+x generate-macos contribute-macos.
• On Windows, you might be presented with a warning that this program is from an unsigned developer. Click "run anyway" to continue.
• On macOS, you might be shown a "permission denied" error. This is because it's a downloaded file and is by default in quarantine. You can remove it from quarantine by running xattr -d com.apple.quarantine contribute-macos. See here for more details: https://superuser.com/questions/526920/how-to-remove-quarantine-from-file-permissions-in-os-x.

Hardware:

• Disable sleep - if your machine is automatically set to sleep or turn hard disks off, it's best to switch these options off.
• Battery - it's best to connect the machine to an outlet, as the setup is computationally intensive and would drain a battery fast.
• Frozen machine - if your computer freezes, don't worry -- the Plumo setup software is using all the threads on the CPU, and it should resolve itself.

If either of these happen, don't worry - stop and restart the contribution program and re-enter the passphrase to continue.

Network:

• If you have issues uploading your contributions, it could be that you have some other processes that are consuming your bandwidth. Make sure to quit any processes such as:
  • A Nest or home camera uploading feeds
  • Torrenting / seeding any files
  • Backup services

Additional Resources

• Plumo Paper
• Plumo Celo Improvement Proposal
• Zero Knowledge Summit Presentation on Plumo
• Transcript of Plumo presentation at Stanford Blockchain Conference
• Zero Knowledge Summit Presentation on the Plumo setup system, Espero